Department of Chemistry, York University 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3 Phone: 416-736-2100 ext. 33852. Fax: 416-736-5936. E-mail: goodings@yorku.ca

GAS-PHASE ION CHEMISTRY IN FLAMES

A burning mixture of a fuel, oxidant and diluent can be stabilized on a burner to produce a stationary flame. Such a premixed flame consists of a preheat zone in which the gases warm up, a bright luminous reaction zone where most of the chemistry takes place, and a hot burnt-gas region which is nearly isothermal. Any flame with a hydrocarbon fuel contains a small concentration of natural flame ions (~10^-8 mole fraction); CH radicals from the fuel and O atoms from the oxidant undergo a chemi-ionization reaction, CH + O = HCO⁺ + e^-, to produce positive ions HCO⁺ and free electrons e^-. Starting with HCO⁺, a series of fast ion/molecule reactions involving many neutral flame species X results in a wide variety of positive flame cations (e.g. HCO⁺ + H₂O = H₃O⁺ + CO). Also, e^- attaches to electronegative species to form negative ions which react with other flame neutrals HY to produce a similar variety of negative flame anions (e.g. OH^- + HY = Y^- + H₂O). These ions can be observed by sampling a flame along its axis through a nozzle into a mass spectrometer.^1,2 The fundamental data consist of profiles of an ion concentration versus distance along the flame axis extending from the burnt gas through the reaction zone into the unburnt gas upstream. Apart from the natural ion chemistry, the flame can be doped with an unlimited variety of flame additives; a gas saturator attached to the burner supplies is used for volatile liquids, or a pneumatic atomizer is employed to introduce an aqueous aerosol of inorganic species such as metallic salts. For some studies, it is advantageous to use hydrogen flames which contain no natural ionization; e.g. the primary flame ionization of metals by thermal (collisional) ionization or chemi-ionization.¹ Very recently, structures and energetics of transition-metal ions and neutral flame species have been calculated using density functional methods.

These methods have been exploited to study a variety of research problems in flames, and their applications to combustion phenomena in general.

• Natural ion chemistry of hydrocarbon flames in which the flame ions are used as a probe of the underlying neutral chemistry of combustion.²

• Combustion of alternative fuels; e.g. methanol.

• Investigation of pollutant formation in flames by nitrogen (NO_x) and sulphur (SO_x).

• Investigation of soot formation in fuel-rich flames; soot suppression by metallic additives.

• Flame-ion chemistry of metals: alkali, alkaline earth, transition metals and lanthanides.¹

• Elimination of free electrons from the burnt-gas region of flames using electronegative additives to promote negative ion formation (electron scavenging).³

• Flame-ion chemistry of non-metals: the halogens, sulphur, silicon and phosphorus.³

References:

1. J.M. Goodings, C.S. Hassanali, P.M. Patterson and C. Chow. A new flame-ion mass spectrometer: chemi-ionization of lanthanum observed in hydrogen-oxygen- argon flames. Int. J. Mass Spectrom. Ion Processes, 132, 83-96 (1994).
2. J.M. Goodings, D.K. Bohme and C.-W. Ng. Detailed ion chemistry in methane-oxygen flames. I. Positive ions. Combust. Flame, 36, 27-43 (1979).
3. J.M. Goodings and C.S. Hassanali. Ion chemistry of phosphorus in hydrocarbon flames. I. Electron scavenging by negative ion formation. Int. J. Mass Spectrom. Ion Processes, 101, 337-354 (1990).

Recent Publications

Q.F. Chen, R.K. Milburn, A.C. Hopkinson, D.K. Bohme and J.M. Goodings. Magnesium chemistry in the gas phase: calculated thermodynamic properties and experimental ion chemistry in H₂-O₂-N₂ flames. Int. J. Mass Spectrom. 184, 153-173 (1999).

Q.F. Chen and J.M. Goodings. Chemical kinetics of lanthanum ionization in H₂-O₂-N₂ flames. Int. J. Mass Spectrom. 188, 213-224 (1999).

J.M. Goodings, J.Z. Guo, A.N. Hayhurst and S.G. Taylor. Current-voltage characteristics in a flame plasma: analysis for positive and negative ions, with applications. Int. J. Mass Spectrom. 206, 137-151 (2001).

J.Z. Guo and J.M. Goodings. Recombination coefficients for H₃O⁺ ions with electrons e^- and with Cl ^-, Br ^- and I ^- at flame temperatures 1820-2400 K. Chem. Phys. Lett. 329, 393-398 (2001).

J.Z. Guo and J.M. Goodings. A density-functional study of the reaction of neutral scandium with water. Chem. Phys. Lett. 342, 169-176 (2001).

J.Z. Guo and J.M. Goodings. A density functional study of the structures and ionization energies of some scandium compounds with hydrogen and oxygen. J. Mol. Struct. (Theochem) 549, 261-273 (2001).

J.Z. Guo and J.M. Goodings. A density functional study of the hydration energies and proton affinities of some scandium compounds with hydrogen and oxygen. J. Mol. Struct. (Theochem) 571, 171-181 (2001).

J.Z. Guo and J.M. Goodings. Scandium ionization mechanisms in H₂-O₂-N₂ flames supported by calculated thermodynamic data. Int. J. Mass Spectrom. 214, 339-348 (2002).

J.Z. Guo and J.M. Goodings. Chemical kinetics of scandium ionization in H₂-O₂-N₂ flames. Int. J. Mass Spectrom. 214, 349-364 (2002).

J.M. Goodings, J.Z. Guo and J.G. Laframboise. Electrochemical diffusion potential in a flame plasma: theory and experiment. Electrochem. Commun. 4, 363-369 (2002).